I just did this with mailman on zcs 6.0.9. Here's a simple 1.2.3 that worked for me..
1. created a new virtual server with centOS 5_64bit and checked the box for mailman during install.
2. added a dns entry for my list server ie. listsrv.mydoma.in (A record only)
3. added the address of the mailman server to the trusted networks- allowed to relay - in my zimbra server's config
4. edited sendmail.mc on the list server to relay mail through my zimbra server for all outgoing email
5. For the convenience of my users, I create an account - list@mydoma.in on my zimbra server for each list which forwards to list@listsrv.mydoma.in
which puts the list in GAL and allows a shorter list email address...
Then it WORKS! yeah!

And just a reminder; you may want to add a robots.txt file to your pipermail's html folder (disallowing webcrawlers) so that your list archives don't get indexed by google and others - unless you want the extra publicity